Exposure control device

ABSTRACT

An exposure control device of a camera by which an appropriate camera-shake limit shutter speed T vf  can be obtained in accordance with a focal length f of a photographing lens, and by which an arbitrary program can be set on the basis of the shutter speed T vf , in which T vf  is obtained by an equation: 
     
         T.sub.vf =log.sub.2 f, 
    
     
         or 
    
     
         T.sub.vf =(log.sub.2 f)·α+β; 
    
     wherein α&lt;1, and α and β are constants which satisfy log 2  f o  =(log 2  f o )·α+β for a predetermined arbitrary focal length f o .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a programmed automatic exposure controldevice, especially to an exposure device by which the effect ofcamera-shake is obviated so that a better quality image is obtained.

2. Description of the Related Art

To make it easier to take good photographs, many cameras today areprovided with a programmed automatic exposure control device(hereinafter abbreviated as exposure control device) by which, accordingto a luminance of an object to be photographed, an aperture and ashutter speed are combined to automatically provide a correct exposure.A conventional exposure control device provided in an interchangeablelens camera is briefly explained as follows.

The prior art exposure control device has a plurality of programdiagrams by which an aperture and a shutter speed are combined to obtainan appropriate exposure. FIG. 4 shows examples of three kinds of programdiagrams, P₁, P₂, and P₃. In FIG. 4, the vertical axis shows an A_(v)value (aperture value), the lateral axis shows a T_(v) value (shutterspeed), and the oblique lines show equivalent exposure values.

In FIG. 4, if the program diagram P₂ is taken as a standard, the programdiagram P₁ shows a line by which a photograph can be taken under acondition in which the shutter speed is raised at the same objectluminance, and the program diagram P₃ shows a line by which a photographcan be taken under a condition in which an aperture is stopped down atthe same object luminance.

In the prior art, a program diagram used for exposure control is chosenfrom among the program diagrams shown in the drawing, according to thefocal length of the interchangeable lens (a photographing lens) used forthe photographing, and where a zoom lens is used in accordance withfocal length changes caused by a rotation of the zoom barrel. Namely, inthe prior art, since a program diagram is not provided for eachphotographing lens having a different focal length, in all cases inwhich a focal length of the photographing lens is longer than apredetermined value f₁, the program diagram P₁ is used, and in all casesin which a focal length of the photographing lens is shorter than f₂(wherein f₂ <f₁), the program diagram P₃ is used; in all the othercases, the program diagram P₂ is used.

In the case of camera-shake, however, it is considered that this hasvery little effect at a shutter speed which is higher than acamera-shake limit speed value determined by a reciprocal number of afocal length of the photographing lens, wherein a dimension is ignored;i.e., the shutter speed is 1/50 sec when the focal length f=50 mm. Withregard to this point, a conventional exposure control device does nothave program diagrams for all photographing lenses having differentfocal lengths, but instead, a photographing condition is divided intozones according to a focal length, and program diagrams are allocated toeach zone. Therefore, a point at which a shutter speed is lowest on theprogram diagram (a starting shutter speed; points shown by T_(v1),T_(v2), and T_(v3) in FIG. 4) is necessarily a common value, forexample, of a plurality of photographing lens for which the programdiagram is applied, and accordingly, the full ability of eachphotographing lens may not be properly utilized. Further, in theconventional program diagram, both the A_(v) value and T_(v) valuechange in accordance with a lowest starting shutter speed (calledT_(vf)) used as a starting point.

SUMMARY OF THE INVENTION

The present invention is intended to solve the above problem, andtherefore, the object of the present invention is to provide an exposurecontrol device in which, not only is an appropriate exposure obtained,but also an image having a better image quality.

To attain this object, the exposure control device of the presentinvention, for determining an exposure according to the use ofphotographing lenses having different focal length, comprises:

means for obtaining a camera-shake limit shutter speed T_(vf) accordingto a focal length F of the photographing lens, by the followingequation:

    T.sub.vf =log.sub.2 f.

According to this construction, for each photographing lens having adifferent focal length, and in the case of a zoom lens, for each focallength changed according to a rotation of the zoom barrel, anappropriate starting shutter speed T_(vf) by which the effect ofcamera-shake is obviated is obtained. Therefore, an exposure conformingvery closely to each photographing lens is determined.

Further, to obtain a more appropriate starting shutter speed T_(vf), thepresent invention comprises a means for obtaining a starting shutterspeed by the following equation, in which the above equation iscorrected (wherein α, β are constants which are determined so as tosatisfy log₂ f₀ =(log₂ f₀)·α+β for a predetermined focal length f₀, andα<1):

    T.sub.vf =(log.sub.2 f)·α+β.

According to this construction, for each photographing lens having adifferent focal length, and in the case of a zoom lens, for each focallength changed according to a rotation of the zoom barrel, anappropriate starting shutter speed T_(vf) at which the effect ofcamera-shake is obviated, is obtained based on the result of acalculation of log₂ f. Further, in the case of a photographing lenshaving a focal length which is shorter than f₀, T_(vf) is corrected to avalue which provides a higher shutter speed than at a value obtainedaccording to log₂. In the case of a photographing lens having a focallength which is longer than f₀, T_(vf) is corrected to a value whichprovides a slower shutter speed than at a value obtained according tolog₂ f. Accordingly, since T_(vf) is determined in such a manner, anexposure suitable for each photographing lens is obtained.

Further, in a program diagram, if a starting shutter speed T_(vf) atwhich the effect of camera-shake is obviated is ensured, an exposure isused which allows photographs to be taken having a greater depth offield namely, stopped down photographing can be carried out.

Still further, the present invention provides an exposure control devicewhich can obtain not only a proper exposure, but also a better imagequality.

To attain this object, the exposure control device of the presentinvention comprises;

a means for setting a camera-shake limit shutter speed (T_(vf)),

a first calculating means for changing at least a shutter speed (T_(vf))according to an object luminance (E_(v)) by a predetermined firstprogram diagram, in accordance with the camera-shake limit shutter speed(T_(vf)) set by the setting means,

a second calculating means for shifting only an aperture value (A_(v))to be stopped down by a degree corresponding to a predetermined objectluminance (E_(v)), while fixing the camera-shake limit shutter speed(T_(vf)) if higher than the camera-shake limit shutter speed (T_(vf))set by the setting means, and

a third calculating means for calculating an aperture value (A_(v)) anda shutter speed (T_(v)) according to an object luminance (E_(v)) by apredetermined second program diagram, if different from a combination ofthe aperture value (A_(v)) shifted by the second calculating means andthe camera-shake limit shutter speed (T_(vf)).

In the operation of the present invention the first calculating meanspreferably changes the shutter speed (T_(v)) to the camera-shake limitshutter speed (T_(vf)) by the first program diagram with the sameaperture value according to the object luminance (E_(v)).

Further, in the operation of the present invention, the thirdcalculating means preferably changes both the aperture value (A_(v)) andthe shutter speed (T_(v)) by the second program diagram according to theobject luminance (E_(v)).

Still further, in the operation of the present invention, the settingmeans of the camera-shake limit shutter speed (T_(vf)) preferablydetermines T_(vf) according to the following equation, for eachphotographing lens having a different focal length, and for a zoom lens,for each focal length changing according to a rotation of the zoombarrel (wherein, in the equation, f is a focal length and α, β areconstants which are set so as to satisfy the equation log₂ f₀ =(log₂f₀)·α+β for a predetermined focal length f₀, where α<1. That is:

    T.sub.vf =(log.sub.2 f)·α+β.

According to this construction, the effect of camera-shake isconsiderably obviated, and photographing becomes possible underconditions in which an aperture value is stopped down by a predeterminedaperture value A_(v) in comparison with a conventional aperture valueA_(v) which corresponds to a predetermined E_(v) value.

Further, since only A_(v) is shifted at T_(vf), the program diagramrises to a right angle at T_(vf). Therefore, when comparing the programdiagram of the present invention (FIG. 3A) and a program diagram havingan inclination as shown FIG. 4 (approximately 60 degrees in this case),an A_(v) value corresponding to E_(v) when a shutter speed T_(v) ishigher than T_(vf), is larger in a program diagram of the presentinvention than in the prior art. Generally, since the definition of alens is better when an aperture is stopped down than when an aperture iswider, a higher quality picture is obtained when the amount of stoppingdown is as large as in a program diagram of the present invention.

Still further, for each photographing lens having a different focallength, and for a zoom lens, for each focal length changed according toa rotation of the zoom barrel, a proper starting shutter speed T_(vf)by, which the effect of camera-shake is obviated, is obtained by theequation:

    T.sub.vf =(log.sub.2 f)·α+β,

whereby a program diagram is provided in which only A_(v) is shifted atT_(vf), by a predetermined E_(v), and both an A_(v) value and T_(v)value change is obtained. Accordingly, since an appropriate value T_(vf)for each photographing lens having a different focal length isdetermined, a program diagram most suitable for photographing isobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from the description ofthe preferred embodiments of the invention set forth below, togetherwith the accompanying drawings, in which;

FIG. 1A is a block diagram showing a function of a CPU as a componentpart of an exposure control device of the present invention;

FIG. 1B is an explanatory drawing of a construction of RAM memory usedin the CPU of FIG. 1A;

FIG. 1C is a drawing showing an example of the use of an accumulatorA_(cc) when obtaining T_(vf) ;

FIGS. 1D through 1F are flow charts of the CPU operation for determiningthe means for obtaining T_(vf) ;

FIGS. 2A through 2D are explanatory drawings of a camera in which theexposure control device of the present invention is applied;

FIG. 3A is an explanatory drawing of a program diagram obtained by anexposure control device of the present invention;

FIGS. 3B through 3D are flow charts of the operation of a CPU foroperating a calculating means for obtaining an exposure condition; and,

FIG. 4 is an explanatory drawing showing a block diagram of a prior artprogram diagram.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention are described with reference toan example in which an exposure control device of the present inventionis provided in an interchangeable lens camera having an automaticfocusing (AF) function and a photographing lens including a lens ROM.Further, the exposure control device of this embodiment is provided witha means for obtaining a camera-shake limit shutter speed T_(vf), foreach photographing lens having a different focal length, and for a zoomlens, for each focal length change in accordance with a rotation of thezoom barrel.

EXPLANATION OF THE CONSTRUCTION OF THE CAMERA

The construction of the above interchangeable camera is described belowwith reference to the drawings. Note that each drawing mentioned in thedescription is roughly drawn and is used only to enable a clearunderstanding of the present invention. Therefore, it is obvious thatthe size, shape, and arrangement of each construction component are notrestricted to the shown examples. The same reference numerals are usedfor the same or corresponding components in each drawing.

FIG. 2A represents a schematic block diagram of the camera.

In FIG. 2A, 31, and a photographing lens 11 is provided on the camerabody 31. In this example, the photographing lens 11 has a single focallength.

The photographing lens 11 is provided with a lens system 15 thatincludes a focusing lens 13 that is movable along the optical axis as anaid to focusing, and a drive force transmittal mechanism 17 thattransmits a drive force to the focusing lens 13 from a drive sourceprovided in the camera body 31. Further, the photographing lens 11 isprovided with a lens ROM (Read Only Memory) 19 which stores aperturevalue information of the photographing lens 11 and focal lengthinformation, and lens side electric contact points 21 for electricallyconnecting the photographing lens 11 and the camera body 31.

The camera body 31 is provided with a finder optical system, such as amain mirror 33, a sub mirror 35, a focusing screen 37 and a pentagonalprism 39, a pickup portion 41 as an aid to automatic focusing, a drivemechanism 43 for driving the focusing lens 13 in the photographing lens11, a photocell 45 for AE (automatic exposure control), a photocell 47provided for TTL regulation when using a strobe light, a centralconcentrating indicator 49 which indicates a condition of the camera, afinder indicator 51 which indicates AF and AE conditions, an internalstrobe 53, a sequence motor 55 for film winding and rewinding, camerabody side electrical contacts 57 which corresponds to the lens sideelectric contact points 21, a release switch 59, and an X contact 61which is used, for example, as a synchronizing contact.

Further, the camera body 31 comprises an IPU (Indication ProcessingUnit) 71 which is a microcomputer controlling the central concentratingindicator 49, a PCU (Power Control Unit) 73 which is an interface forthe pickup portion 41, and controls the sequence motor 55, the AF motor43, and an aperture and a shutter release magnet, and has an E² PROM73a, a DPU (Data Processing Unit) 75 which is a microcomputer carryingout photometry and control of the finder indicater 51, and a CPU(Central Processing Unit) 77 which is a microcomputer for a centralcontrol. The CPU 77 controls the IPU 71, the PCU 73, the DPU 75, and thelens ROM 19 that is provided in the photographing lens 11.

EXPLANATION OF MEANS FOR OBTAINING T_(vf)

The means for obtaining a camera-shake limit shutter speed (T_(vf)) forthe above camera is now described. In this embodiment, T_(vf) isobtained by the equation T_(vf) =(log₂ f)·α+β, and therefore, a methodfor taking in focal length information used in this equation isdescribed below.

<Taking In Focal Length Information>

In the camera shown in FIG. 2A, information stored in the lens ROM 19and showing a focal length of the photographing lens is taken in to theCPU 77 through the electric contacts 21 and 57, regardless of the typeof lens that is used. Although no problem arises when the photographinglens has only one focal length, if the photographing lens is a zoomlens, the focal length value changes in accordance with the rotation ofthe zoom barrel. Therefore, in the lens ROM 19 of the zoom lens,information corresponding to focal length changes caused by such a zoombarrel rotation is stored. This information is read as described below.

FIG. 2B is a rough drawing of a zoom lens that is attachable to thecamera body 31, cut along the optical axis of the lens. Thephotographing lens 81 comprises, in addition to the components providedin the photographing lens 11, a zoom barrel 83, a brush 83a fixed to thezoom barrel 83 and moved by the rotation of the zoom barrel 83, and azoom code plate 83b in slidable contact with the brush 83a. FIG. 2Cshows the electric parts provided in the lens 81 comprising the zoomcode plate 83b, the lens ROM 19, the electric contacts 21 provided inthe lens 81, and a distance code plate 85. These parts are mounted in acylindrical part of the photographing lens in such a manner that eachcode plate 85 is wound along the periphery of the cylindrical part 84.

FIG. 2D is a perspective view on an enlarged scale, showing a part of azoom encoder comprising the zoom barrel 83, the brush 83a, and the zoomcode plate 83b.

When the zoom barrel 83 is rotated, the focal length of thephotographing lens 81 is changed, and at the same time, the brush 83a ismoved backward and forward while in contact with the zoom code plate83b, along the longitudinal direction of the zoom code plate 83b shownin FIG. 2C, and stopped at the position at which the rotation of thezoom barrel is stopped. In this embodiment, four distribution patternsextending from the lens ROM 19 and shown by a₁, a₂, a₃, and a₄ areprovided on a face of the zoom code plate 83b in contact with the brush83a. Note that the number of distribution patterns decides the degree ofresolving power shown by a focal length changed by the rotation of thezoom barrel 83, and this number of distribution patterns may be changedaccording to a desired design and is not restricted to this embodiment.One of the four distribution patterns, in this case the distributionpattern a₄, is an earth line. The other patterns a₁ through a₃ areformed in such a manner that the patterns are made wide or narrow inaccordance with a position thereof along the longitudinal direction ofthe zoom code plate. As shown in FIG. 2D, the brush 83a has fourcontacts b₁ through b₄ corresponding to the distribution patterns a₁through a₄, which are electrically connected to each other. Althoughmany different shapes of contacts are possible, each contact in thisembodiment has a dual construction which improves the reliability of thecontact. Also, each contact is constructed to come into contact with thecorresponding distribution pattern at the wide portion of thedistribution pattern, and in particular, the contact shown by b₄ isalways in contact with the distribution pattern a₄ (the earth line).

In the construction decribed above, when the brush 83a is moved inaccordance with a rotation of the zoom barrel 83, at a position z₁ ofthe zoom code plate 83b (shown in FIG. 2C), each contact of the brush83a and the distribution patterns a₁, a₂, and a₃ are in contact at a₁and a₂, and are not in contact at a₃. Since the distribution patterns a₁through a₃ are pulled up on the lens ROM 19, and each contact of thebrush is connected to the earth line through the distribution patterna₄, a voltage condition of each distribution pattern a₁, a₂, a₃ at thez₁ position is (0,0,1) in the order of a₁ ˜a₃ (0 denotes a low level).The voltage condition is (0,1,0) at a position z₂, (1,0,0) at positionz₃, and is (0,0,0) at position z₄. An electric signal obtained bychanging the voltage condition is used directly as an address of thelens ROM 19, so that information corresponding to focal lengthinformation stored in the lens ROM 19 is read. The information read fromthe ROM and showing the focal length is transmitted to the CPU 77 in thesame way as for a photographing lens having a single focal length.

<Explanation of an Approximate Calculation of log₂ f>

A process for obtaining a camera-shake limit shutter speed T_(vf), byusing information showing a focal length obtained as described above, isexplained below. In this embodiment, T_(vf) is first obtained as aresult of an approximate calculation of log₂ f. The means for obtainingT_(vf) is mainly constructed by the lens ROM 19 and the CPU 77, whichhas a construction as described later.

FIG. 1A is a block diagram showing a function of the CPU 77 comprising aROM 77a which stores a program for operating the means for obtainingT_(vf), 77b shows a comparing means 77b, a calculating means, a RAM 77d,and an input and output port 77e. The CPU 77 carries out informationtransfers to and from the lens ROM 19, the PCU 73, and the DPU 73through the input and output port 77e. In this embodiment, the RAM 77dcomprises an EA register operating as a 16 bit register, B and Cregisters which are 8 bit registers, and an accumulator A_(cc), as shownin FIG. 1B. Note that the EA register is divided when used, i.e., isused as an EAH register which is a high-order 8 bit register, and as anEAL register which is a low-order 8 bit register. The process for theapproximate calculation depends upon a degree of accuracy (resolvingpower) of T_(vf). In this embodiment, since the luminance of an objectto be photographed is currently obtained at every 1/8 E_(v), the processfor obtaining T_(vf) is described below as an example correspondingthereto. Further, each bit of the accumulator A_(cc) in an approximatecalculation of log₂ f is weighed as shown in FIG. 1C.

A process for the approximate calculation of log₂ f is described belowwith reference to a flow chart of the CPU 77 for operating the means forobtaining T_(vf), as shown in FIGS. 1D through 1F.

The CPU 77 takes information stored in the lens ROM 19 and showing afocal length of the photographing lens into the EA resister (step 201).Regarding the information showing a focal length, as described above,where the photographing lens has a single focal length, information ofthe focal length is taken in, and for a zoom lens, informationcorresponding to a focal length decided by a rotation of the zoom barrelis taken in. Note that, in this embodiment, since the informationshowing a focal length stored in the lens ROM 19 is compressed to 8 bitsto be stored, the information is changed to a type of integer to bestored when stored in the EA register of the CPU 77.

This compression and changing are briefly described below. In 8 bitinformation stored in the lens ROM 19, the low order 2 bits are a firstbit group having a weighting of 2² and 2⁴ from the low order side, andthe high order 6 bits are a second bit group having a weighting 2⁰, 2¹,2², 2³, 2⁴, and 2⁵ from the high order side. A change to an integer ismade by multiplying the sum of values of each bit of the second bitgroup by the product of values of each bit of the first bit group, andthen multiplying the result by a predetermined constant.

Subsequently, with regard to the information stored in the EA registerfrom the lens ROM 19 and showing a focal length, it is determinedwhether or not data of a high-order 8 bit of the EA register, i.e., dataof the EAH register, is 00H (H means a hexadecimal indication, and isthe same hereinafter). The result of this comparison, shows whether thefocal length of the present photographing lens is more than, equal to orless than 256 mm (2⁸).

If EAH=00H in step 203, the B register is set to 08H, and data of theEAL resister is stored in the accumulator A_(cc) (steps 205, 207). Onthe other hand, if EAH≠00H, the B register is set to 10H, and data ofthe EAH register is stored in the accumulator A_(cc) (steps 209, 211).

Then, the data of the B register is decremented by 1, and the resultantvalue is stored in the B register (step 213). The content of theaccumulator A_(cc) is shifted leftward, i.e., the content is shifted by1 bit from a low-order to a high-order (step 215), and then it isdetermined whether or not a carry (CY) in this shift is 1 (step 217). Ifthe carry (CY) is 0, the process is returned to step 213, and steps 213and 215 are repeated until CY becomes 1.

If CY=1, i.e., if the bit of the highest order of focal lengthinformation stored in the EA register appears, it is determined to whatcolumn this bit corresponds. The number of the column is shown in thepresent value of the B register. Therefore, in the above process, thevalue of the integer part of the approximate calculation of log₂ f isobtained.

To obtain a decimal part (1/8 step) of the approximate calculation oflog₂ f, it is determined whether or not B<06H (step 219). In thiscomparison, when the value is more than or equal to 6, the valueobtained by subtracting 6 from the value of the B register is stored inthe C register B (step 221), and the value of the EA register is thenshifted rightward, i.e., is shifted by 1 bit from a high-order bit ofthe EA register to a low-order bit of the register (step 223), so that avalue obtained by subtracting 1 from the C register is stored in the Cregister (step 225). If C=OFFH is not true, the process is returned tostep 223, and steps 223 and 225 are repeated until C=OFFH, i.e., C=-1.Accordingly, when C=OFFH, i.e., when information of the fifth low-orderbit counted from the highest order column of the information showing afocal length stored in the EA register is stored in the 0 bit of the EAregister, 1 is added to the value of the EA register, and the resultantvalue is stored to the EA register (steps 227 and 229).

At this time, the value of the EA register is shifted rightward by 1bit, and by this shift, in the 0 bit of the EA register, information ofthe fourth bit, which is counted in a low-order direction from thehighest order column of the information stored in the EA register andshowing a focal length is stored (step 231) and 1 is added to the EAregister, whereby the resultant value is again stored in the EA register(step 233). In steps 229 and 233, 1 is added to the values in the EAregister means to correct the columns of 1/16 E_(v) and 1/32 E_(v), andthus the approximate calculation of log₂ f is carried out in a highaccuracy. Note that, although 1 is added to values of fourth and fifthbits counted in a low-order direction from the highest column of theinformation showing the focal length, this is because T_(vf) correspondsto the 1/8E_(v) step, and if a resolving power of the T_(vf) is changed,the bit position to which 1 is added is changed.

The value of the EA register is further shifted rightward by 1 bit (step235), and as a result, in 0 through 2 bits of the EA register (to beexact, the EAL register), information corresponding to the decimal part(1/8) of a result of the approximate calculation of log₂ f is stored.Information of the lower order 3 bits, i.e., the 0 through 2 bits of theEAL register, is stored in the accumulator A_(cc) (step 237).

Then, a process for obtaining a final result of the approximatecalculation of log₂ f is carried out. As described above, an integerpart of the result of the approximate calculation of log₂ f isinformation stored in the B register. Therefore, taking a logical sum ofthe information stored in the B register and the information stored inthe accumulator A_(cc) and showing the decimal part, the result of theapproximate calculation of log₂ f is obtained. To adjust the position ofthe figure of the information of the B register to the position of thefigure of the information of A_(cc), the information of the B registeris shifted leftward by 3 bits (step 239), and the logical sum is taken(step 241). Accordingly, the result of the approximate calculation oflog₂ f is obtained.

At step 219, which is in the middle of the approximate calculation, ifB<06H, it is determined whether or not B<05H (step 251). In thiscomparison, if B is larger than or equal to 5, this means that B isequal to 5, and as a result, the process at step 229 is carried out. Onthe other hand, if B<05H, it is determined whether or not B<04H (step253). In this comparison, if B is larger than or equal to 4, this meansthat B is equal to 4, and as a result, the process at step 233 iscarried out. Accordingly, when B is 5, similar to the case in which B is6, a process for improving an accuracy of the approximate calculation iscarried out (steps 229 and 233). On the other hand, when B is 4, aprocess in which 1 is added to the fourth bit from the highest bit toimprove an accuracy of the approximate calculation is carried out (step233).

In step 253, when B<04H, the process at step 237 is carried out.

The result of the approximate calculation of log₂ f, obtained asdescribed above, is a value generally known as a camera-shake limitshutter speed, and therefore, is preferably used as a starting point ofa shutter speed T_(vf) when determining a program diagram for eachphotographing lens having a different focal length. Therefore, althoughthe result of the approximate calculation can be used directly asT_(vf), in this embodiment, the following correction is further made tothe result of the approximate calculation.

<Correction of the Result of the Approximate

Calculation of log₂ f>

The reason why it is preferable to correct the result of the approximatecalculation of log₂ f is as follows:

When taking good pictures under normal conditions, if the focal lengthof the photographing lens is short, since the depth of field is long,the aperture is preferably relatively wide and the shutter speed ishigh. On the other hand, if the focal length is long, since the depth offield is short, the aperture is preferably relatively closed and theshutter speed is low. Further, the effect of camera-shake on aphotographing lens having a short focal length is greater than theeffect thereof on a photographing lens having a long focal length, sincethe total length of the photographing lens having the short focal lengthis short, and the total length of the photographing lens having a longfocal length is long.

In this regard, it would appear that the effect of camera-shake on aphotographing lens having a 300 mm focal length, for example, is 10times greater than that on a lens having a 30 mm focal length, but thisis not true, since the effect of camera-shake on the former lens is lessthan 10 times that on the latter lens, because the former lens is easierto be hold. Therefore, when using a photographing lens having a focallength f of 24 mm, for example, rather than using a shutter speed ofabout 1/24 sec, it is preferable to use a higher shutter speed (1/30 secor 1/60 sec). Similarly, when using a photographing lens having a focallength f of 300 mm, for example, rather than use a shutter speed ofabout 1/300 sec, it is possible to use a lower shutter speed withoutworrying about the effect of camera-shake.

Accordingly, in this embodiment, a result of the approximate calculationof log₂ f is corrected according to the equation (1) below, so that theresult of the correction is a starting shutter speed T_(vf) of theprogram diagram for each photographing lens having a different focallength.

    T.sub.vf =(log.sub.2 f)·α+β            (1)

wherein α<1, and α and β are constants which satisfy log₂ f₀ =(log₂f₀)·α+β for a predetermined focal length f₀.

Note that, in the case, this correction means of the approximatecalculation of log₂ f is mainly constructed by the CPU 77 and the PCU73. The reason for the use of the PCU 73 is that this embodiment isconstructed in such a manner that the constants α and β in the equation(1) are stored in the E² PROM 3a provided in the PCU 73, so that the CPU77 takes in these constants when necessary. Although a construction inwhich α and β are stored in the program ROM 77a of the CPU 77, forexample, without the E² PROM 73a, is possible, if the E² PROM isprovided, an advantage by which α and β are easily changed, even if thedesign is changed, is obtained.

A correction for a result of the approximate calculation of log₂ f isdescribed below with reference to an example wherein f₀ =250 mm.

First, when f₀ =250 mm, α and β satisfying an equation of log₂ 250=(log₂250)·α+β are determined. Although α and β can be various values, in thisembodiment, it is assumed that α=3/4 and β=2. These values are stored ina predetermined adress of the E² PROM 73a.

The subsequent correction process is carried out after step 241 shown inFIG. 1F as described below.

The CPU 77 takes α and β stored in the E² PROM 73a of the PCU 73 intothe calculating means (step 243), and then uses a result of theapproximate calculation of log₂ f stored in the A_(cc) to carry out thecorrection calculation of the above equation (1), to obtain a startingshutter speed T_(vf) (step 245).

An effect of the correction process for the result of the approximatecalculation of log₂ f carried out as described above is described belowwith reference to an example.

(1) When a photographing lens has a focal length f=250 mm, a result ofthe approximate calculation of log₂ 250 is log₂ 250≈8, since 250≈2⁸=256, and a result of the correction according to equation (1) is

    T.sub.vf =8×3/4+2=8.

This corrected value is equal to the result of the approximatecalculation.

(2) When a photographing lens has a focal length f=1000 mm, althoughlog₂ ≈1000 10, since 1000≈2¹⁰ =1024, the result of the approximatecalculation is corrected according to equation (1), so that the startingshutter speed is changed to a lower value.

    T.sub.vf =10×3/4+2=9.5

(3) When a photographing lens has a focal length f=30 mm, although log₂30≈5, since 30≈2⁵ =, the result of the approximate calculation iscorrected according to equation (1), so that the starting shutter speedis changed to a higher value.

    T.sub.vf =5×3/4+2=5.75.

Therefore, it can be understood that, by carrying out a correction asdescribed above, the starting shutter speed is compressed from bothsides of the the long and short focal lengths, with f=250 mm as thecenter. Note that, although T_(vf) for each focal length f is compressedfrom f=250 mm, the f₀ is not restricted to 250 mm, but may be 125 mm or500 mm.

EXPLANATION OF CALCULATION MEANS FOR EXPOSURE CONDITION

The calculation means used for obtaining an appropriate exposurecondition (an aperture value A_(v) and a shutter speed T_(v)) for eachobject luminance (E_(v)) is described below.

The starting shutter speed T_(vf) obtained as described above is ashutter speed by which it is ensured that the effect of camera-shake isobtained. Therefore, an exposure condition for obtaining a good pictureis preferably shifted relative to T_(vf), in such a manner that anaperture value A_(v) is stopped down by a predetermined value (thisshift amount is shown by A_(vs) (FIG. 3A). Accordingly, in thisembodiment, the program diagram is determined in such a manner that,when E_(v) is larger than or equal to A_(vmin) +T_(vf) (A_(vmin) meansan open F value of the photographing lens), only A_(v) is shifted by apredetermined E_(v) value, and then both A_(v) and T_(v) are changed.FIG. 3A is an explanatory drawing of a program diagram in which theA_(v) value is shifted by (E_(vj) -E_(vi)). In this drawing, a solidline shown by I is the program diagram, and a broken line III shows aprogram diagram of a prior art. The program diagram shown by I isobtained, after obtaining T_(va) and T_(vb) at the same E_(v) valueaccording to the following equations (a) and (b), by choosing the highervalue thereamong as the T_(v) value:

    T.sub.va =E.sub.v ·Y/X+δ                    (a)

    T.sub.vb =T.sub.vf ·A.sub.v =E.sub.v -T.sub.vb    (b).

Note that X and Y are constants defining inclinations of programdiagrams, where X=8 and Y=3 in the case of FIG. 4. Also, δ is the T_(v)value when E_(v) =0, and is obtained by the following equation (c).

    δ=(1-Y/X)·T.sub.vf -Y(A.sub.vmin +A.sub.vs)/X(c).

<Construction of Calculating Means>

In this embodiment, the calculating means for determining an exposurecondition according to a program diagram shown by I is constructedmainly by the CPU 77 and the PCU 73. The reason for the use of the PCU73 is that this embodiment is constructed in such a manner that theconstants X and Y determining an inclination of a program diagram andthe constant A_(vs) denoting a shift amount are stored in the E² PROM73a provided in the PCU 75, and thus the CPU 77 takes in these constantswhen necessary. Another embodiment may be constructed in such a mannerthat X, Y, and A_(vs) are previously stored in the program ROM 77a, forexample, so that the PCU 73 is not used. But if the E² PROM is used asdescribed above, an advantage is obtained in that the constants can beeasily changed when a design change is made.

In a calculation in which an exposure condition is determined, the EAHregister of the RAM 77d of the CPU 77 is used as a register for T_(v),the EAL register is used as a register for A_(v), and A_(cc) is used asa register for T_(vf). The calculating process and the comparing processused in the operation are carried out by the calculating means 77c andthe comparing means 77d, respectively.

<Procedure for Determining an Exposure Condition>

FIGS. 3B through 3D are flow charts of the CPU for operating thecalculating means for determining an exposure condition.

The result of the approximate calculation of log₂ f is stored in theT_(vf) register (step 301), and then the data of the T_(vf) respectfullyis stored in the T_(v) respectfully (step 303). Then an object luminanceE_(v), is obtained from luminance information in the DPU 75 (shown inFIG. 2A), so that a calculation of E_(v) -T_(v) is carried out by usingE_(v), and the result of the calculation is stored in the A_(v) register(step 305). Subsequently, it is confirmed whether the data in the A_(v)register is a value in a range between the open aperture (A_(vmin)) andthe minimum aperture (A_(vmax)) of the photographing lens used. If thedata value is within that range, the data of the A_(v) register ismaintained as it is, and if the data value is outside that range, thedata is replaced by an appropriate one of A_(vmin) and A_(vmax),according to the conditions (steps 307 through 331). Note thatinformation showing A_(vmin) and A_(vmax) is stored in the lens ROM ofthe photographing lens, and thus this information is taken in to the CPUfrom the lens ROM.

The T_(v) value is obtained, on the basis of the A_(v) value determinedas described above, and stored in the T_(v) register (step 315), and itis confirmed whether the data in the T_(v) register is a shutter speedwhich is within a capacity of the shutter mechanism of the camera. Ifthe shutter speed is within that capacity, the data in the T_(v)register is maintained as it is, and if the shutter speed is outsidethat capacity, the data is replaced by an appropriate one of the maximumshutter speed (the shortest exposure time) T_(vmax) and the minimumshutter speed T_(vmin) (steps 317 through 323). By carrying out theprocess of steps 307 through 323, the diagram shown by II in FIG. 3A canbe obtained. The T_(v) value (corresponding to T_(vb) of equation (b))determined as described above, is stored in the A register (step 325).

Next, the CPU 77 reads the constants X and Y defining inclinations ofthe program diagrams and the constant A_(vs) showing a shift amount fromthe E² PROM 73a of the PCU 73 (step 327). These X, Y and A_(vs) arevalues corresponding to the design of the camera, respectively. In thepresent embodiment, X=8, Y=3, and A_(vs) is a value corresponding to 1E_(v).

Then, δ is obtained by equation (c), and is stored in the B register(step 329).

The T_(va) value is obtained by the equation (a), and is stored in theT_(v) register (step 331).

Then, the data (T_(va)) of the T_(v) register is compared with the data(T_(vb)) of the A register. When T_(v) >A, the data of the T_(v)register is maintained as it is. If T_(v) <A, the data of the A registeris stored in the T_(v) resister (step 333 and 335). In the process sofar, the larger of the values T_(va) and T_(vb) obtained by theequations (a) and (b) is stored in the T_(v) register.

The process shown by steps 337 through 365, carried out after step 335,is used to determine an appropriate T_(v) value and A_(v) value whileconsidering the maximum shutter speed T_(vmax) and the minimum shutterspeed T_(vmin) of the camera, and the A_(vmin) and A_(vmax) of thephotographing lens in use.

First, if the T_(v) value stored in the T_(v) register at present islarger than the maximum shutter speed T_(vmax) of the camera, the valueof the T_(v) register is replaced by T_(vmax) (step 339), and thus theA_(v) value is obtained by using this T_(vmax) (step 341). Then, if theA_(v) value obtained as described above is larger than A_(vmax), thevalue of the A_(v) register is replaced by A_(vmax), and if the A_(v) issmaller than A_(vmax), the data is maintained as it is (steps 343 and345). The A_(v) value at this time is determined as the aperture to beused. Then the T_(v) value is newly obtained by using the A_(v) value inthe A_(v) register, and this T_(v) value is stored in the T_(v) resister(step 347). T_(v) is again compared with T_(vmax) (step 349), and ifT_(v) is larger than T_(vmax), the value of the T_(v) register isreplaced by T_(vmax), and thus, this value is determined as the shutterspeed. Conversely, if T_(v) is not larger than T_(vmax), the value inthe T_(v) register is determined as the shutter speed. In step 337, ifthe T_(v) value is less than or equal to T_(vmax), and the sum of theT_(v) value and the open aperture value A_(vmin) is more than or equalto the present E_(v) value of the object, the process of steps 341through 351 is carried out.

On the other hand, when the present T_(v) value is judged to be lessthan T_(vmax) at step 337, and the sum of the T_(v) value and the openaperture value A_(vmin) is judged to be less than the E_(v) value, thevalue of the A_(v) register is replaced by A_(vmin) (step 355), and thenthe T_(v) value is newly obtained under this condition, and this T_(v)value is stored in the T_(v) register (step 357). If this T_(v) value isless than 0, the value of the T_(v) register is replaced by 0 (step361), and if more than 0, the value is maintained as it is, and thus thevalue of the T_(v) register is compared with the minimum shutter speed(the longest exposure time is obtained) T_(vmin) (step 363). When T_(v)<T_(vmin), the T_(vmin) value is determined as the shutter speed, and ifT_(v) <T_(vmin), the T_(v) value stored in the T_(v) register at presentis determined as the shutter speed. Note that the aperture is open(A_(vmin)) in each case.

OTHER EMBODIMENTS

The present invention is not restricted to the above embodiment, and canbe modified as described below.

In the embodiment described above, for each photographing lens having adifferent focal length, and for each focal length change in accordancewith a rotation of the zoom ring of a zoom lens, the limit shutter speedT_(vf) at which the effect of camera-shake is obviated is obtained by acalculation, and program diagrams of the present invention are obtained.The present invention, however, can be applied to many differentexposure control devices, regardless of the method of setting acamera-shake limit shutter speed.

As shown in FIG. 4, for example, even if any program diagram is chosenaccording to the focal length of the lens used, i.e., even if T_(vf) ispreset, the present invention can be applied. In this case, if theconstruction of the device is as described below, then, for example, thedetermination of program diagrams (exposure condition) can be carriedout in the same way as in the above embodiment.

First, each starting shutter speed T_(v1), T_(v2), and T_(v3) of programdiagrams P₁ ˜P₃ in FIG. 4, and threshold focal lengths f₁ and f₂ forcontrolling each photographing lens having a different focal length withthree program diagrams P₁ through P₃, are stored in the E² PROM 73a ofthe PCU 73, respectively. If a focal length f_(n) of the photographinglens used is taken in to the CPU 77, as in the above embodiment, thisf_(n) and the above f₁ and f₂ are compared with each other. Thiscomparison can be easily carried out by using the comparing means 77b.Then, using an electrical signal obtained by this comparison, acorresponding starting shutter speed is read from T_(v1), T_(v2), andT_(v3) in the E² PROM 73a. The read shutter speed is determined asT_(vf), and then a process after step 303 shown in FIG. 3B is carriedout. According to this operation, even if a camera-shake limit shutterspeed T_(vf) is a common value for several different photographinglenses, a program diagram starting from a shutter speed at which theeffect of camera-shake is obviated, and which enables photographing witha large depth of field, is obtained.

The procedure in which T_(vf) is obtained by the approximate calculationof log₂ f when setting T_(vf) is not restricted to the flow chart shownin FIGS. 1D through 1F, and may be obtained by other procedures.Further, the procedure for determining an exposure condition is notrestricted to the flow chart shown in FIGS. 3B through 3d, and may beobtained by other procedures.

The exposure control device of the present invention can be applied notonly to a camera having the construction described in the aboveembodiment, but also to other cameras, such as an electronic stillcamera and an interchangeable lens camera having only an AE function.

As understood from the above description, according to the exposurecontrol device of the present invention, for each photographing lenshaving a different focal length, and, for a zoom lens in which the focallength changed in accordance with a rotation of a zoom ring, anappropriate starting shutter speed T_(vf) at the effect of camera-shakeis obviated is obtained every time a program diagram appropriate to eachphotographing lens is determined.

Accordingly, not only an appropriate exposure but also a better qualityimage is obtained.

Further, according to the exposure control device of the presentinvention, for each photographing lens having a different focal length,and for a zoom lens in which the focal length is changed in accordancewith a rotation of a zoom ring, an appropriate starting shutter speedT_(vf) at which the effect of camera-shake is obviated is obtained onthe basis of a result of a calculation of log₂ f. Further, the T_(vf)for a photographing lens having a focal length shorter than f₀ iscorrected to a value corresponding to a faster shutter speed than avalue obtained according to log₂ f, and the T_(vf) for a photographinglens having a focal length longer than f₀ is corrected to a valuecorresponding to a slower shutter speed than a value obtained accordingto log₂ f. Still further, on the basis of the T_(vf) determined asdescribed above, a program diagram appropriate to each photographinglens is determined.

Accordingly, not only an appropriate exposure but also a better qualityimage is obtained.

Still further, according to the exposure control device of the presentinvention, since an exposure control is carried out according to aprogram diagram starting from a shutter speed at which the effect ofcamera-shake is obviated, and enabling photographing with a long depthof field, not only an appropriate exposure but also an image having abetter quality is obtained.

Although the embodiments of the present invention have been describedherein with reference to the accompanying drawings, obviously manymodifications and changes may be made by those skilled in this artwithout departing from the scope of the invention.

We claim:
 1. An exposure control device of a camera, comprising:meansfor detecting a focal length f of a photographic lens attached to acamera body of said camera; means for calculating an exposure forobtaining a camera-shake limit shutter speed T_(vf) according to saidfocal length f of said photographic lens, according to an equationT_(vf) =(log₂ f)·α+β, wherein α<1, and α and β are constants satisfyingan equation log₂ f₀ =(log₂ f₀)·α+β for a predetermined focal length f₀.2. An exposure control device of a camera, comprising:a photographiclens that is detachably mounted to a camera body, focal lengthinformation of said photographic lens being stored in a lens ROMprovided in said photographic lens; and means for calculating anexposure for obtaining a camera-shake limit shutter speed T_(vf)according to said focal length f of said photographic lens, according toan equation: T_(vf) =log₂ f, said exposure calculating means readingsaid focal length information from said lens ROM for an exposurecalculation.
 3. An exposure control device of a camera, comprising:azoom lens, a focal length of said zoom lens being changed in accordancewith a rotation of a zoom barrel, said zoom lens being detachablymounted to a camera body of said camera, said zoom lens comprising acode plate having a focal length code provided along a direction ofmovement of said zoom barrel corresponding to a stop position of saidzoom barrel, and means for reading said focal length code when said zoombarrel is stopped; and means for calculating an exposure for obtaining acamera-shake limit shutter speed T_(vf) according to a focal length f ofsaid zoom lens, according to an equation: T_(vf) =log₂ f, said exposurecalculating means taking in said focal length code read by said codereading means for an exposure calculation.
 4. An exposure control deviceof a camera, comprising:a zoom lens, comprising:a code plate having afocal length code provided along a direction of movement of a zoombarrel; and means for reading said focal length code when said zoombarrel is stopped, a focal length f of said zoom lens being changed inaccordance with a rotation of a zoom barrel associated with said zoomlens, said zoom lens being detachably mounted to a camera body of saidcamera; and means for calculating an exposure for obtaining acamera-shake limit shutter speed T_(vf) according to a focal length fread by said code reading means of said zoom lens, according to anequation: T_(vf) =log₂ f, wherein said focal length code read by saidcode reading means is provided to said exposure calculating meansthrough a contact between lens electric contacts provided in said zoomlens and body electric contacts provided in said camera body.
 5. Anexposure control device of a camera, comprising:a zoom lens,comprising:a code plate having a focal length code provided along adirection of movement of a zoom barrel; and means for reading said focallength code when said zoom barrel is stopped, a focal length f of saidzoom lens being changed in accordance with a rotation of a zoom barrelassociated with said zoom lens, said zoom lens being detachably mountedto a camera body of said camera, wherein said zoom code plate is fixedto a barrel which is formed of an insulating material, and which facessaid zoom barrel along the direction of movement of said zoom barrel,said focal length code formed on said zoom code plate having wiringpatterns formed by electric conductors insulated from each other, oneend of said wiring patterns being connected to a plurality of inputports of a lens ROM, each wiring pattern having a narrow width portionand a wide width portion to obtain predetermined focal lengthinformation by a combination of said narrow width and wide widthportions, in a direction perpendicular to a direction of movement ofsaid zoom barrel, said code reading means having brushes provided with aplurality of contacts which can be independently slidably contacted withsaid wide width portions without coming into contact with said narrowwidth portions of each of said wiring patterns, said contacts beingfixed to said zoom barrel, whereby a constant voltage is applied to aportion between said brushes and said wiring patterns, so that said lensROM senses a voltage of each of said wiring patterns to output focallength information corresponding to the voltage of each of said wiringpatterns; and means for calculating an exposure for obtaining acamera-shake limit shutter speed T_(vf) according to a focal length fread by said code reading means of said zoom lens, according to anequation:

    T.sub.vf =log.sub.2 f.


6. An exposure control device of a camera, comprising:a photographiclens that is detachably mounted to a camera body, focal lengthinformation of said photographic lens being stored in a lens ROMprovided in said photographic lens; and means for calculating anexposure for obtaining a camera-shake limit shutter speed T_(vf)according to said focal length f of said photographic lens, according toan equation: T_(vf) =log₂ f, said exposure calculating means readingsaid focal length information from said lens ROM for an exposurecalculation, wherein said focal length information of said lens ROM isprovided to said exposure calculating means through a contact betweenlens electric contacts provided in said photographic lens and bodyelectric contacts provided in said camera body.
 7. An exposure controldevice of a camera according to claim 5, wherein two pairs of said zoomcode plate and said brush are provided at different positions.
 8. Anexposure control device for use in a camera, comprising:means forsetting a camera-shake shutter speed T_(vf) ; first calculating meansfor changing at least a shutter speed T_(v) in accordance with an objectluminance E_(v) by a predetermined first program diagram when a shutterspeed T_(v) is smaller than said camera-shake limit shutter speed T_(vf)set by said camera-shake limit setting means: second calculating meansfor shifting an aperture value A_(v) so as to be stopped down by aconstant value corresponding to a predetermined object luminance, whilefixing said shutter speed T_(v) to said camera-shake limit shutter speedT_(vf) when said shutter speed T_(v) is equal to said camera-shake limitshutter speed T_(vf) ; and third calculating means for changing saidaperture value A_(v) and said shutter speed T_(v) in accordance withsaid object luminance E_(v) by a predetermined second program diagramafter said aperture value A_(v) is shifted and said camera-shake shutterspeed T_(vf) is fixed by said second calculating means.
 9. An exposurecontrol device for use in a camera, comprising:means for setting acamera-shake shutter speed T_(vf) first calculating means for changingat least a shutter speed T_(v) in accordance with an object luminanceE_(v) by a predetermined first program diagram when a shutter speedT_(v) is smaller than said camera-shake limit shutter speed T_(vf) setby said camera-shake limit setting means; second calculating means forshifting an aperture value A_(v) so as to be stopped down by a constantvalue corresponding to a predetermined object luminance, while fixingsaid shutter speed T_(v) to said camera-shake limit shutter speed T_(vf)when said shutter speed T_(v) is equal to said camera-shake limitshutter speed T_(vf) and third calculating means for changing saidaperture value A_(v) and said shutter speed T_(v) in accordance withsaid object luminance E_(v) by a predetermined second program diagramafter said aperture value A_(v) is shifted and said camera-shake shutterspeed T_(vf) is fixed by said second calculating means, wherein saidcamera-shake limit shutter speed T_(vf) is obtained by an equationT_(vf) =log₂ f, wherein f is a focal length of a photographic lensassociated with said camera.
 10. An exposure control device for use in acamera, comprising:means for setting a camera-shake shutter speed T_(vf)first calculating means for changing at least a shutter speed T_(v) inaccordance with an object luminance E_(v) by a predetermined firstprogram diagram when a shutter speed T_(v) is smaller than saidcamera-shake limit shutter speed T_(vf) set by said camera-shake limitsetting means: second calculating means for shifting an aperture valueA_(v) so as to be stopped down by a constant value corresponding to apredetermined object luminance, while fixing said shutter speed T_(v) tosaid camera-shake limit shutter speed T_(vf) when said shutter speedT_(v) is equal to said camera-shake limit shutter speed T_(vf) thirdcalculating means for changing said aperture value A_(v) and saidshutter speed T_(v) in accordance with said object luminance E_(v) by apredetermined second program diagram after said aperture value A_(v) isshifted and said camera-shake shutter speed T_(vf) is fixed by saidsecond calculating means; and exposure calculating means for obtainingsaid camera-shake limit shutter speed T_(vf) by an equation T_(vf)=(log₂ f)·α+β, according to the focal length of said photographic lens,wherein α<1, and α and β are constants which are set to satisfy log₂ f₀=(log₂ f₀)·α+β for a predetermined focal length f₀.
 11. An exposurecontrol device of a camera according to claim 8, wherein said firstcalculating means changes said shutter speed T_(v) to said camera-shakelimit shutter speed T_(vf) with the same aperture value A_(v), accordingto the object luminance obtained by said predetermined first programdiagram.
 12. An exposure control device of a camera according to claim8, wherein said second calculating means changes the aperture valueA_(v) so as to be stopped down to an open aperture value A_(vmin) bysaid object luminance E_(v), while fixing said shutter speed T_(v) tosaid camera-shake limit shutter speed T_(vf), when said shutter speedT_(v) is equal to said camera-shake limit shutter speed T_(vf).
 13. Anexposure control device of a camera according to claim 8, wherein saidthird calculating means changes both said aperture value A_(v) and saidshutter speed T_(v) according to said object luminance E_(v) obtained bysaid predetermined second program diagram.
 14. An exposure controldevice of a camera according to claim 8, wherein said photographing lensis detachably mounted to said camera body, and focal length informationof said photographing lens is stored in a lens ROM provided in saidphotographing lens, and said exposure calculating means reads said focallength information from said lens ROM for an exposure calculation. 15.An exposure control device of a camera according to claim 8, whereinsaid photographing lens is a zoom lens in which the focal length ischanged in accordance with rotation of a zoom barrel, said photographinglens being detachably mounted to a camera body, said zoom lenscomprising a code plate having a focal length code provided along adirection of movement of said zoom barrel, corresponding to a stopposition of said zoom barrel, and code reading means for reading saidfocal length code corresponding to said stop position when movement ofsaid zoom barrel is stopped, said exposure calculating means taking insaid focal length code position read by said code reading means for anexposure calculation.
 16. An exposure control device of a cameraaccording to claim 15, wherein said focal length code position read bysaid code reading means is taken in to said exposure calculating meansthrough a contact between lens electric contacts provided on saidphotographing lens and body electric contacts provided on said camerabody.
 17. An exposure control device of a camera according to claim 15,wherein said zoom code plate is fixed to a barrel formed by aninsulating material, facing said barrel along the direction of movementof said zoom barrel,said focal length code formed on said zoom codeplate having wiring patterns formed by electric conductors insulatedfrom each other, one end of said wiring patterns being connected to aplurality of input ports of said lens ROM, each wiring pattern having anarrow width portion and a wide width portion to obtain predeterminedfocal length information by a combination of said narrow width portionand said wide width portion in a direction perpendicular to a directionof movement of said zoom barrel, said code reading means being providedwith brushes having a plurality of contacts which are independently inslidable contact with said wide width portions without coming intocontact with said narrow width portion of each wiring pattern, and arefixed to said zoom barrel, whereby a constant voltage is applied to aportion between said brushes and said wiring patterns, so that said lensROM reads said focal length code position according to a change of eachof said wiring patterns.
 18. An exposure control device of a cameraaccording to claim 14, wherein said focal length information of saidlens ROM is taken in to said exposure calculating means through acontact between lens electric contacts provided in said photographinglens and body electric contacts provided in said camera body.
 19. Anexposure control device of a camera according to claim 17, wherein twopairs of said zoom code plate and said brush are provided at differentpositions.